1. Field of the Invention
This invention relates to an optical signal recording medium.
2. Description of the Prior Art
Optical recording media, in particular, optical disks, which have been rapidly developed recently, are expected to be used for a variety of purposes and applications as the market expands. Optical disks may be roughly divided into two categories, that is, read-only disks and rewritable disks. The former type is produced by transferring patterns of information formed on a stamper to a transparent substrate by injection molding, and evaporating aluminum on the information-transferred. Although the stamper is expensive, a large number of disks (more than 10,000) having the same information can be produced by using a single stamper. Thus, the former type is suitable to mass-production for a manufacturer. The music compact disc (CD), is a typical example of the former type.
This method, however, is not suited to small scale productions such as the production of personal information files or personal music software. This is because the production of the stamper requires an extremely precise cutting machine and a large scale developing process. Not only is the stamper expensive, but it also lacks flexibility in software production because the software and the stamper must be produced in different companies or locations. These are factors which raise the production cost per signal recording medium.
On the other hand, the rewritable optical disk is produced by forming an optical recording film on a surface of a transparent substrate on which shallow grooves for tracking have been formed, and providing a protective layer opposing the surface with a gap therebetween. This construction is suitable for personal use, but the medium in the unrecorded state is, in itself, expensive. Besides, since the same laser beam is used for recording and reproducing, if the same position is repeatedly subjected to the laser beam for a long time, the recorded signal will gradually deteriorate, or the output for recording may be used by mistake when reproducing, thereby damaging the recorded signal. Hence, it is not an inexpensive and highly reliabile optical recording medium. Moreover, since the reflectivity from the recording surface is low, being less than 40%, and the thickness is more than twice that of a CD, it requires a reproducing apparatus that is different from CD players. The rewritable optical disk is disclosed, for instance, in Japanese Unexamined Patent Publications No. 59-24692 and No. 60-35054, while a read-only optical disk is explained, for example, in NIKKEI ELECTRONICS, Aug. 17, 1981.
Incidentally, a method of bleaching a signal-recorded film, and reproducing pits on the recorded film is disclosed in U.S. Pat. No. 4,270,130. Since this method includes recording a signal after forming a flat reflection film, it is accompanied by energy loss due to heat conduction by the reflection layer at the time of recording, and a large amount of power is needed for recording. Besides, in order to obtain an error rate similar to that of a CD, the optical path from the substrate surface to the recorded surface must be set to the same distance as in a CD, and for this purpose, a sandwich structure is indispensable. As a result, the total thickness becomes about twice that of a CD.